This invention relates to a device intended for connection of body members of pressure vessels, especially to permit the rapid removal and fitting of lids or covers of pressure vessels, such as the covers of nuclear reactor pressure vessels.
It is commonly desired to rapidly fit or dismantle the covers for the pressure vessels or pressurized light water reactors, which frequently operate at pressures of approximately 150 bars. For this purpose, it is necessary to rapidly disconnect or connect the flange of the cover from or to that of the vessel, these flanges being connected together by a very large bolting system. This system is generally composed of stud bolts set in the flange of the pressure vessel, onto which there are screwed nuts which press the cover against the vessel and squeeze an intermediate seal. In a pressurized water reactor of 900 Mw, for example, these stud-bolts may be 58 in number and typically have a diameter of approximately 150 mm and a length of approximately 1 meter.
The very high tightening torque which must be exerted does not generally permit the nuts to be tightened up in the conventional manner. One possible technique which has long been used, for example for tightening up of turbine half-casings, consists of pulling the stud-bolt by using a hydraulic jack which bears against the flange of the cover, and of then bringing up the nut against the cover. After the bolt has been released, the nut holds the tension which was temporarily exerted by the jack.
However, in the case of a structure such as pressurised water reactor, the use of a sealing joint or gasket which is of metal, and, therefore, very hard, means that it is necessary to fit or to dismantle almost simultaneously all the nuts, if it is desired to avoid deformations in the flanges, which would be induced by the reaction force of the seal.
The need for covers of nuclear reactor pressure vessels to be capable of being rapidly dismantled and refitted is due to the fact that it is desirable to limit the maximum possible extent the shut-down time of the power station. A power station (or section thereof) of 900 Mw capacity costs approximately U.S. $0.25/Watt installed capcity. The cost of the power station when shut-down can be estimated as approximately U.S. $250,000 a day. Since this shut-down normally occurs once per year, it is easy to understand the advantage of requiring only one day, instead of three days, for dismantling or refitting the cover of the pressure vessel.
Whereas the forces to be produced may be very high in order to stress the stud-bolts, for example of the order of 500 tons per bolt in a pressurized water reactor of 900 Mw capacity, the distance of travel involved is small, of the order of a few millimeters.
The devices at present in use for the dismantling or refitting of the covers of nuclear reactor pressure vessels are essentially based on the use of high pressure hydraulic jacks.
This technique does not provide a satisfactory answer, in that there are several major disadvantages:
since the stud-bolts are normally very close to one another, this restricts the permitted bulk opposite to a bolt and requires the use of jacks and stud-bolts of contorted shape. In particular, the stud-bolts have to be specially machined to permit the use of this type of mechanism, and their replacement must be planned in advance if this technique is envisaged for use latter on an existing power station;
there is a risk of escape of oil from the jacks or hydraulic circuits, which can pollute the equipment, the water of the pressure vessel and the environment. The risk of these leakages in increased by the fact that the mechanism is used only occasionally;
certain components, in particular the elastomer seals of the jacks, cannot withstand the high radiation doses originating from the reactor core, nor the temperature of the cover when the reactor is in operation, which means that the mechanism has to be mounted and dismantled during each recharging of the reactor;
the removal (sometimes necessitated) of a portion of the bolts before the cover is raised, becomes a very difficult matter, in view of the fact that the jacks bear above the bolts.